hey m,
FIRST OF ALL, POROSITY in alum is from hydrogen. second, is what are ya cleanin it with? what is yer CUP dia.? what flavor filler metal are you using? either 4043 or 5356 works best for 6061. if yer gonna get it annodized, use 5356. you should be usin ac with high freq, but also dc ( i forget what polarity) will work for thin gage alum. isoprophol alcohol is the best, safest cleaner to use. make sure you ss wire brush the area to be welded to remove alum oxide. make sure you dont have no finger prints on yer tungsten and alum. cause it has oils in it. DO NOT add filler metal until you see shiny molten alum. (if yer using filler rod). are ya gettin any black soot? also you cant scratch start alum., you will get contamination. i hope yer filler rod is no bigger than .032. what is yer gas flow, shouldnt be very high so you dont get the venturi effect which starts to suck in attmosfere. maybe like 15-20. i have never welded such a small size alum. but im tryin to help. lemme know how ya make out. im not a great teacher, but if i was there, we could make it work. make sure you arent using acetone or some other residew leavin substance.
hope this helps,
rich

the biggest problems w/ porosity on aluminum is either dirty aluminum, or a problems w/ the gas flow. make sure there are no drafts in the area of the weld 'cause that blows the shielding gas away. most people always want to ventilate a welding area, but in the case of MIG and TIG there is a such thing as too much ventilation. and if the alum. is dirty, oily, or still has the oxide coating on it, you'll end up w/ porosity. it's not like welding steel...welding alum. is a lot like surgery...everything is cleaned, and even if it looks clean, it's cleaned again.

you never said if you were butt welding the tubing, or fillet welding it. if you're butt welding it, your problems could be because you need an argon purge on the inside of the tubing to keep the atmosphere from touching the inside of the bead. which brings up another thing...clean the inside of the tubing if you're butt welding it. that oxide is all over any exposed surface of alum. make sure you're using a stainless steel brush of some kind when stripping off the oxide.

Here is a good link with a condensed article by Howard Cary, my choice for welding textbook authors.

http://www.welding.org/newsletters/summer1998/aluminum.html


Observations:

MCWelding notes correctly that most porosity in aluminum welds are caused by hydrogen mainly, and surface contaminants solidifying inside the weldment. How it's getting there is the big question. My first thought in this regard is that your tube diameter is pretty small. If your welder is moving a bit too slowly, the excessive heat buildup can be a cause of hydrogen absorption.

Secondly a small diameter, aluminum tube is tough to weld in position. Often times its difficult to keep a consistent torch angle <which is critical>, this in turn can have negative effects on both penetration and gas coverage. If you have access to a rotating positioner, those welds might be made at top dead center, with a more consistent torch angle and at a much quicker pace <with practice> thus lowering heat input and boosting production time. You may also want to try a gas lens style collet to improve argon coverage, if you do this you must check periodically to make sure the screen mesh is in good conditions as AC operations can quickly deteriorate gas lenses.

Surface preparation: All cutting fluids or rough edges need to be removed. All surfaces must be prepped as outlined; this includes the butting faces of the tube. Isopropyl Alcohol, TriChloroEthane or Acetone are all suitable cleaning agents that leave no residue according to the majority of aerospace specifications. Each of the afore mentioned cleaning agents have safety and health and environmental considerations which must be taken into account. You may obtain an MSDS (material data safety sheet) wherever you purchase your cleaning fluids.

Other variables which require diligent consideration include Tungsten selection <Zirconium or Cerium> and cleanliness (the electrode must be perfectly clean for each arc strike). Any contamination must be reason to stop work and immediately redress the electrode. A dirty tungsten can make every weld in a series fail X-ray. Also Balance control settings too far toward max penetration (overextending dwell time on the straight polarity side of the half cycle) can cause porosity and poor surface appearance, this is especially true if less than perfect pre-weld surface prep is in evidence.

After considering all this stuff if you have more questions just ask.